I'm thinking about adding a 2" receiver hitch on the back of my 2011 Rockwood 1809s TT. FR advises that this will void the warranty (expired anyway) as may cause problems with TT frame. Company line is they don't recommend...
There is a welding shop nearby that can build a custom hitch receiver welded directly to the TT frame. They advise replacing the 4" back bumper with a heavier gage steel and have lots of experience.Does anyone have any experience with this type installation? Is there much risk in the added weight of bicycles or < 150lb causing damage to TT frame or structure?

My first advice is to do a search on this forum. The majority of posts will show you why this is a bad idea. A really bad idea!

Your trailer is a mini lite. Meaning it has the weakest and lightest frame possible. The warranty is just one issue. What is really problematic is the stress caused to the frame with having a bike rack extend several feet out. These trailers bounce like crazy in the back as you go down the road. That is a lot of stress and its not just the static weight of the bikes. If you do it, better make sure that this welding shop somehow strengthens the frame to allow for 150+ lbs bouncing as you are moving.

Thanks... this is why I need to hear from folks with more experience with this issue. More than likely I will carry 2 bikes so weight will probably be closer to 90lb. Still I don't want any problems with my TT investment...

I'm sorry to be the bearer of bad news. Trust me, if I felt I could do it, it would be the very next thing I do. We routinely travel with two adult and three children's bikes for the races we do as a family in the summer. I carry all inside the trailer. They are the last things in and the first to come out. Its a pain, but it is what it is.

My neighbor did that to his new outback. I went over and cranked the bumper back to where it belonged and re-welded it to the frame. Was going to install one on my bumper but he tested it out on his first. Needless to say I don’t have one on mine

As stated, "It is a REAL bad idea", can not be repeated often enough.
The problem is NOT the weight; but where it is located that is the problem. It is located some distance from the attachment point. This makes a lever (like a teeter totter) with the fulcrum at the welds. The bouncing at the bikes is quite severe and that up/down travel will crack any weld. Worse yet; if the attachment welds are strong enough; that torque is transferred to the frame at the aft rear axle weld points. Frame cracks are a certainty.

FR tried beefing up its frame on several TT models in 2010 and 2011 to add a generator/cargo tray to their line. They looked WONDERFUL and I considered upgrading my camper to one of the newer models till I ran the numbers. (After 3 years it is "just how I like it" and would need to start over again with a new one).

Must not have worked out very well since in mid-year 2011 they stopped offering it on their travel trailers. My guess is frame cracks and not "buyer demand" was the real reason for the cancellation of the option.

I have never had one on a trailer but I did have one on the reciever of my ATV. They become a giant lever. I could never keep the thing in one piece. That experience is enough to tell me I won't put one on my travel trailer. The leverage there is even greater.